The invention relates to a shock sensor utilizing liquid, in particular to a shock sensor which is responsive only to shocks of magnitude in excess of a given value.
A number of applications are found for a shock sensor. For example, it may be mounted on an automobile to detect a collision accident to expand an air bag for protection of the life of the occupants. Alternatively, it may be mounted on a glass window or door in order to issue an alarm when the window is destroyed by a thief, by detecting the shocks applied to the pane. A conventional shock sensor comprises a mass of metal which is resiliently biased and which closes an electrical circuit by contact with an oppositely disposed electrode when the mass has moved more than a given distance. This type of shock sensor which depends on the electrical contact is susceptible to the deterioration of contact, resulting in a poor reliability and a reduced useful life. In addition, a resonance may be caused by mechanical vibrations, giving rise to a malfunctioning. Another difficulty is experienced in increasing the sensitivity by reducing the resilience of the spring and increasing the weight of the metal mass, in that a massive metal body can not be properly retained by a spring having a reduced resilience.
Another type of shock sensor is known having a vessel in which a liquid passageway is formed for receiving a body of liquid so that when the vessel is subjected to a shock lengthwise of the passageway, the liquid moves relative to the passageway so as to permit a detection of the shock applied, using the velocity of the liquid flow as a variable. This has an advantage of increasing the detection velocity by detecting the velocity of flow rather than a displacement of a mass. Further advantages are obtained in that the reliability is increased by avoiding a mechanical contact and that unlike a mass-spring system, the resonance which results from vibrations can be shifted out of the frequency band used. Additionally, the magnitude of the shock applied can be determined.
Where a shock sensor is mounted on an automobile for detection of shocks, the air bag must not be expanded in response to shocks of small magnitude since otherwise the expanded bag will block the sight of the driver, causing an even greater accident. When driving on an uneven road, the automobile is subjected to small shocks perpetually, and in such instance, the detection of shocks is not required or preferably should be avoided. This is true also in a collision where the running velocity is not high enough to cause a loss of human life or a serious damage. In view of these facts, it is desirable that only shocks of magnitude in excess of a given value be detected. Similarly, in the application to the detection of a glass window or door being destroyed, it is desired that the shock sensor does not operate in response to shocks occurring when opening or closing the window or door or shocks of the wind, but responds only in response to shocks which are produced when the pane is being destroyed. In consideration of these and other factors, the present inventors has previously proposed the provision of threshold presetting means in a shock sensor utilizing a body of liquid, in which a motion of the liquid is only permitted in response to shocks of magnitude in excess of a given value.
The threshold presetting means may be disposed intermediate the ends of the liquid passageway and constructed such that the liquid can move past the threshold presetting means when the energy of the liquid tending to move in response to a shock exceeds a given value. However, when a shock sensor provided with such threshold presetting means is subjected to a shock in the opposite direction to cause a movement of the liquid in the opposite direction or away from the threshold presetting means and is subjected to a second shock immediately thereafter, there will occur a movement of the liquid unrestricted by the threshold presetting means, thereby resulting in a detection of the shock. Such an event will be experienced on an automobile having its shock sensor arranged so as to detect a collision with a forwardly located object, when a shock of relatively small magnitude is applied to the automobile immediately after it has been bumped from behind to a slight degree. Vibrations experienced on an uneven road may cause a movement of the liquid in a direction away from the threshold presetting means, which may prevent a proper functioning of the latter. In addition, where a continual vibration of the shock sensor occurs when driving on an uneven road, for example, accompanying a corresponding oscillation of the internal liquid, a resonance may be caused, whereby the shock sensor may operate improperly to detect it as a large shock. Where it is desired to provide a uniform detection of shocks from an angular range or from all directions, the liquid passageway is constructed in the form of a plurality of radially extending branch paths which cover the angular extent or all the directions, with their branch points communicating with a common path. A proper detection can be obtained in this instance if the movement of the liquid which occurred in one of the branch paths is transmitted in its entirety to the common path, but the shock to be detected may not be detected if a part of the liquid flow is diverted to another or other branch paths. The above discussions also apply in the application of the shock sensor to the detection of destruction of a glass window or door wherein the application of a shock of a relatively small magnitude applied after the liquid has moved away from the threshold presetting means without passing therethrough may result in a false detection of the destruction of the pane. In addition, a resonance may be caused in the pane in response to vibrations which are caused by the wind.
Therefore, it is an object of the invention to provide a shock sensor utilizing liquid which does not respond to shocks of magnitude less than a given value.
It is another object of the invention to provide a shock sensor of the type described in which a movement of the liquid is prevented for shocks from any direction which are of magnitude less than a given value.
It is a further object of the invention to provide a shock sensor which blocks an ingress of air into the liquid passageway when permitting a movement of the liquid during a return flow of the liquid after its movement has occurred.
It is an additional object of the invention to provide a shock sensor which prevents air from remaining in the liquid passageway when the vessel assumes a proper position subsequent to its positional adjustment.